In certain industrial operations hydrochloric acid is formed as a by-product of chlorination. There is usually no immediate market for the hydrochloric acid. The lack of a market makes hydrochloric acid production problematic in that it cannot be dumped into sewers and wastewater outlets without costly neutralization. It has been customary in industry to utilize electrolysis of hydrochloric acid to overcome disposal issues. Electrolysis of brines and hydrochloric acid are completed within an electrolytic cell with an anode and a cathode. An electrical potential is established between the anode and the cathode whereby the negatively charged chloride ions are attracted to the anode. Monoatomic chlorine atoms combine at the anode to form diatomic chlorine molecules. In such chlorine production the chlorine molecules form gas bubbles on the surface of the anode and chlorine is recovered as a gas. Monoatomic hydrogen atoms combine at the cathode to form diatomic hydrogen molecules as gas bubbles on the surface of the cathode and hydrogen is also recovered as a gas.
As a consequence of the significant increases in energy costs and the increased scarcity of industrial fuel supplies, intensive research in the field of electrolysis has been performed in order to reduce the amount of power used in industrial electrolysis processes. The cost of electrolysis is proportional to the voltage at which the electrolysis is effected. Thus, it is desirable to reduce the amount of voltage at which a solution is electrolyzed to as low of a value as possible. Methods and apparatuses described in the prior art for achieving a low hydrogen over-voltage include the following:
U.S. Pat. Nos. 1,915,473 and 4,116,804 describe various methods for producing “Raney nickel”, an active porous nickel which provided one of the first alternative cathodic materials to mild steel. This alternative material is used to produce exceptionally low over-voltage as compared to steel cathodes.
U.S. Pat. No. 4,401,529 describes an improved hydrogen evolution cathode of the same type as “Raney nickel” with addition of molybdenum to produce NiMo “Raney nickel” surface.
U.S. Pat. Nos. 4,425,203, 4,430,186 and 4,466,868 describe the addition of small amounts of from 1 to 5 percent of Ti to NiMo alloy which was found to reduce over-voltage for hydrogen evolution in alkaline solutions in comparison with that of the NiMo alloy in Raney nickel.
U.S. Pat. No. 4,975,161 describes hydrogen evolution cathodes produced by thermal decomposition of a mixture of elements of the groups IB, IIB, IIIA, IVA, VA, VIA, VIB and VIII of the Periodic Table.
U.S. Pat. No. 5,395,422 describes the process of producing nanocrystalline metallic powders containing Ni, Co, and Fe or mixtures thereof while the alloying element is one or more transition metals such as Mo, W or V, to be used as catalytic materials for hydrogen evolution.
U.S. Pat. Nos. 5,324,395 and 5,492,732 describe plasma spray techniques for obtaining durable low hydrogen over-voltage cathodes bearing a coating which has an outer layer with at least 10 percent of cerium oxide and at least one non-noble Group VIII metal.
U.S. Pat. No. 5,433,797 describes another type of cathode for hydrogen evolution, the design of which is based on the use of nanocrystalline metals of average grain size and less than about 11 nanometers of tertiary and quaternary NiFeCr and NiFeCrMn alloys. This cathode is obtained by electrodeposition using pulsating direct current regimes.
U.S. Pat. No. 3,616,445, describes electrodes made of titanium, known as “dimensionally stable anodes” (DSA). The introduction of DSA into the processes of chlorine, chlorate and hypochlorite production by the electrolysis of brine has led to significant decreases in energy costs. DSA are made of titanium coated with a thermally prepared mixture of TiO2 and RuO2. DSA are corrosion resistant, selective to chloride ion oxidation and exhibit a high catalytic activity. However, the coating must be routinely replaced.
U.S. Pat. No. 3,950,240 describes a procedure of obtaining a catalytic coating of tin oxide with niobium and a relatively small amount of noble metal oxides. This procedure is advantageous over DSA as smaller amounts of expensive noble metal oxides are required.
U.S. Pat. Nos. 4,511,442, 4,107,025 and 4,007,107 describe metal coated anodes. U.S. Pat. No. 5,587,058 describes electrodes with better corrosion resistance than DSA in the process of chlorine production.
U.S. Pat. Nos. 3,486,994 and 4,210,501 describe production of chlorine by electrolysis of hydrochloric acid in an electrolytic cell having anolyte and catholyte chambers. U.S. Pat. No. 3,242,065 describes production of chlorine from hydrochloric acid using an electrolytic cell with the graphite cathode attached to the frame of the cell. U.S. Pat. No. 5,770,035 describes a method for the production of chlorine from hydrochloric acid in a electrolytic cell, with a cathode compartment equipped with a gas diffusion cathode fed with air, enriched air or oxygen.
U.S. Pat. No. 4,959,132 describes a process of fabrication of thin, electronically conductive, high-surface area film formed on both sides of a membrane to form a bipolar structure useful for electrolysis of hydrochloric acid.
U.S. Pat. No. 5,580,437 describes a particular anode for conversion of hydrochloric acid into chlorine gas using an electrochemically active material of tin, germanium or lead, or mixtures thereof.
U.S. Pat. No. 6,066,248 describes a process for the electrolysis of aqueous hydrochloric acid solution in an electrochemical flow reactor with a solid polymer electrolyte membrane, a platinum-based anode, a cathode and backings.
A number of commercial processes of electrolysis of hydrochloric acid for production of chlorine have been developed (see e.g. F. R. Minz, “HCl—electrolysis—Technology for Recycling Chlorine”, Bayer AG, Conference on Electrochemical Processing, Innovation & Progress, Glasgow, Scotland, UK Apr. 21–23, 1993). A currently employed commercial electrochemical process is known as the Uhde process. In this process aqueous HCl solution of approximately 22 percent is fed at 65 to 70 degrees Celsius into an electrochemical cell into both the anodic and cathodic compartments which are divided by a diaphragm made of special type of PVC cloth. Graphite is used as electrode material for both, anode and cathode (bipolar electrode). Exposure to a direct current in the cell results in an electrochemical reaction and a decrease in HCl concentration of up to 17 percent with the production of chlorine gas in the anodic compartment and hydrogen in the cathodic compartment. Both the anode and cathode side of a graphite bipolar electrode, undergo severe destruction after operating for some time in a cell for hydrochloric acid electrolysis (F. M. Berkey, “Electrolysis of Hydrochloric Acid Solutions”, Ch. 7 in “Chlorine its Manufacture, Properties and Uses”, Ed. J. S. Sconce, New York, Reinhold Publishing Corp., 1962). The use of DSA in this process is not recommended since the titanium substrate undergoes significant corrosion in concentrated HCl at high temperatures and the electrode becomes unusable after a short operating time. Other materials stable in hydrochloric acid like platinum group metals are excessively expensive.
The present invention provides an electrolytic cell which utilizes electrodes exhibiting high stability in acidic media, particularly in concentrated hydrochloric acid. The electrodes have a corrosion rate of less than 2 ìm per year. The electrodes also exhibit low over-voltage for hydrogen and chlorine evolution. Methods of their use as cathodes and anodes in production of chlorine by electrolysis of hydrochloric acid and brine are also provided.